The present invention relates to a pellicle to be mounted on a mask or reticle (hereinafter both will be referred to as a mask) used in production process of integrated circuits with a purpose of preventing attachment of foreign substances, and its production method.
The patterning is carried out by exposing a semiconductor wafer having a resist material coated thereon in photolithography to be used in a process of producing integrated circuits. If e.g. scars or foreign substances are present on a mask to be used, the scars or foreign substances are printed on the wafer together with the pattern, thus causing short circuit or breakage of wire. Accordingly, a method of mounting a pellicle on one side or both sides of the mask as a foreign substances guard on the surface of the mask, has been employed. In the present specification, a pellicle comprises a flat pellicle sheet and a pellicle frame having a predetermined thickness so as to isolate the pellicle sheet from the mask, and has a shape of a container by attaching the pellicle sheet on the upside of the pellicle frame.
Conventionally, as a pellicle, one having an outline as illustrated by a front view and a side view in FIG. 1, comprising a pellicle frame 1 made of e.g. aluminum and a pellicle membrane 2 (corresponding to the above pellicle sheet) made of nitrocellulose or a fluororesin and having a thickness of from several nm to several xcexcm bonded to the pellicle frame by means of an adhesive 3, is used, and this is fixed on a mask so that a pattern on the mask is covered, as disclosed in JP-A-63-15250 or JP-A-3-39963.
Along with high-integration of LSI in recent years, a technique to draw circuit patterns with thinner lines has been required in photolithography, and accordingly use of an exposure light source emitting light having a shorter wavelength has been promoted. For example, for a light source of a stepper for photolithography, a light source emitting light having a shorter wavelength such as a KrF excimer laser (wavelength 248 nm), a ArF excimer laser (wavelength 193 nm) or a F2 laser (wavelength 157 nm) becomes used, in addition to conventional g-line (wavelength 436 nm) and I-line (wavelength 365 nm).
As pellicle membrane material resistant to such a short-wavelength light source, a fluorine-containing polymer having a relatively low absorption in the ultraviolet region, such as CYTOP (tradename, manufactured by Asahi Glass Company, Limited) which is a commercially available fluorine-containing resin or Flon AF (tradename, manufactured by Du Pont Kabushiki Kaisha) which is a fluorine-containing resin, has been known. However, such a fluorine-containing polymer shows practical light transmittance and durability to light when the exposure light source is a KrF excimer laser or a ArF excimer laser, but it has inadequate light transmittance and undergoes decomposition by laser irradiation when the exposure light source is a F2 laser.
On the other hand, as a material transmitting I-line and g-line, a synthetic quartz glass has been known, and JP-A-8-160597 proposes to attach a synthetic quartz glass sheet as a pellicle sheet to a pellicle frame and to use the assembly as a pellicle.
However, studies by the present inventors have clarified that there are several problems when a synthetic quartz glass is used as a material of the pellicle sheet.
A first problem is that a pellicle sheet made of a synthetic quartz glass has a certain degree of thickness, whereby it has to have a high light transmittance depending upon the thickness.
Further, a second problem is as follows.
Conventionally, as a pellicle membrane, a polymer membrane which is slightly larger than the pellicle frame is prepared by spin coating, this membrane is bonded to an opening of the pellicle membrane by means of e.g. an epoxy type adhesive, and a surplus portion is cut off to produce a pellicle.
On the other hand, in a case where a synthetic quartz glass is used as the pellicle sheet, it is necessary to prepare a synthetic quartz glass sheet having its size adjusted to the size of an opening of the pellicle frame by cutting.
As a method of cutting a glass sheet of e.g. synthetic quartz glass, a method of applying a cutting line on the surface of the glass sheet while supplying a glass cutting oil thereto and splitting the glass along the cutting line, may be mentioned. In this method, cracks may form along the cutting line and the cut surface or microfragment (hereinafter sometimes referred to as cullet) may peel off during splitting of the glass sheet.
In order to avoid attachment of swarf or cullet generated from the cut surface on the glass sheet after cutting the glass sheet, the glass sheet surface may be covered with e.g. a paper sheet, or the glass sheet surface may be cleaned after cutting, however, it is difficult to obtain a glass sheet having no cullet or the like attached thereto by these means, because cullet will be generated from the cracks on the cut surface even after cleaning. Further, the cracks may cause fracture when an impact is applied thereto due to subsequent handling.
Further, a third problem is as follows.
If there is a dispersion of the thickness of the pellicle sheet on the plane, the light path of refracted light changes, whereby the position of a transcription pattern shifts, and accordingly no acceptable lithography an be carried out. In FIG. 2 is conceptually shown this state, wherein an exposure beam 305 being transmitted through a pellicle sheet 304 having an inclined surface is refracted in a direction indicated by the arrow.
On the other hand, a synthetic quartz glass sheet is processed by slicing a shaped synthetic quartz glass block. A pellicle sheet is required to have no scar on the surface to be an obstacle to exposure, and in order to use the synthetic quartz glass sheet as a pellicle sheet, it is required to polish the synthetic quartz glass sheet after slicing and process it into a predetermined thickness. This polishing is usually carried out by means of a double surface polishing apparatus.
The processing of the pellicle sheet comprises a lapping step as rough grinding and a polishing step as mirror finish, and a double surface polishing apparatus is used in both steps. As the double surface polishing apparatus, a polishing apparatus as illustrated in a partial side view of FIG. 3 and in a partial oblique view of FIG. 4 has been known. This polishing apparatus comprises a bottom wheel 306 and an upper wheel 307, and is used for polishing by the surface of e.g. cast iron itself in the lapping step and by means of the polishing cloth in the polishing step.
Further, gears are formed on the periphery of a carrier 308 as a work holder. Between a solar gear 309 and an internal gear 310 in the main body of the apparatus, the carrier 308 is set, and the upper and bottom wheels or one of the two wheels is rotated so that the carrier 308 revolves, and both surfaces of a work piece 311 can be polished simultaneously. Here, a supply port of an abrasive material is provided on the upper wheel side, although not shown in the figure, to supply an abrasive material to the work piece 311 during polishing.
However, in the case of processing by means of such an apparatus, the polishing amount per unit time hardly be uniform as between the periphery and the center portion of the work piece, and it tends to be difficult to supply the abrasive material so that it uniformly acts on the plane of the work piece, and accordingly it tends to be difficult to uniformly polish the plane of the work piece.
Further, a fourth problem is as follows.
Namely, in JP-A-8-160597, the thickness of the pellicle sheet is preferably from 0.01 to 0.5 mm, however, if the synthetic quartz glass is processed into this thickness, there are significant problems in fracture and breakage during processing or during its use.
It is an object of the present invention to provide a pellicle and its production method to overcome the above problems.
The present invention provides a pellicle which comprises a pellicle frame and a pellicle sheet made of a synthetic quartz glass, attached to an opening of the pellicle frame, wherein the pellicle sheet is made of a synthetic quartz glass having a OH group concentration of at most 100 ppm and containing substantially no oxygen deficient defect. Particularly, the present invention provides the above pellicle, wherein the pellicle sheet is made of a synthetic quartz glass having a OH group concentration of at most 10 ppm, containing substantially no oxygen deficient defect and having an internal transmittance of at least 80%/cm at a wavelength of 157 nm.
According to Embodiment 1 of the present invention, the above pellicle wherein the pellicle sheet has a centerline average surface roughness Ra of at most 0.5 xcexcm on the side face in an area within 10 xcexcm from the surface, is provided. Further, the above pellicle wherein the dispersion of the sheet thickness of the pellicle sheet is within xc2x10.3 xcexcm/150 mm, is provided.
Further, the above pellicle wherein the pellicle sheet is attached to the pellicle frame so that the outline of the pellicle frame extends beyond the outline of the pellicle sheet, and the width of the extension is from 0.05 to 1 mm, is provided.
Still further, a method for producing a pellicle comprising a pellicle frame and a pellicle sheet made of a synthetic quartz glass, attached to an opening of the pellicle frame, which comprises a step of polishing a synthetic quartz glass sheet having an outline dimension larger than the dimension of the pellicle frame by at least 5 mm, and cutting the synthetic quartz glass sheet into a predetermined dimension to produce the pellicle sheet, and a method for producing a pellicle comprising a pellicle frame and a pellicle sheet made of a synthetic quartz glass, attached to an opening of the pellicle frame, which comprises a step of polishing a synthetic quartz glass sheet in such a state that a dummy portion to be processed is attached to the periphery of the synthetic quartz glass sheet to produce the pellicle sheet, are provided.